1. Field of the Invention
The invention relates to a double-roller-type tripod constant-velocity joint used in, for example, a power transmission mechanism of a vehicle.
2. Description of the Related Art
A double-roller-type tripod constant-velocity joint (hereinafter, simply referred to as “constant-velocity joint”) has been conventionally used as, for example, a drive shaft that transmits rotary driving force for vehicle travel, to vehicle wheels. A conventional constant-velocity joint includes a conventional roller unit 130 illustrated in an example in FIG. 6. As illustrated in FIG. 7A, a tripod shaft portion 11A is passed through the roller unit 130, that is, the roller unit 130 is fitted onto the tripod shaft portion 11A. In the roller unit 130, as illustrated in a sectional view in FIG. 6 in which the roller unit 130 has been assembled, an inner roller 134 is placed into an outer roller 131 from a side of the outer roller 131, on which a rib 131B is not formed. A plurality of needles 133 is disposed in an annular space formed between the outer roller 131 and the inner roller 134. A ring groove 131A is formed in an inner wall of the outer roller 131. A snap ring 135 is fitted in the ring groove 131A, so that the snap ring 135 is fitted to the outer roller 131. One end face of the outer roller 131 is provided with the rib 131B that extends radially inward and that has an annular shape. The rib 131B retains the inner roller 134 and the needles 133 so that the inner roller 134 and the needles 133 do not slip off the outer roller 131 from the one end face side. The snap ring 135 is fitted to the other end face side-portion of the outer roller 131. The inner roller 134 and the needles 133 are retained by the snap ring 135 so as not to slip off the outer roller 131 from the other end face side.
In the roller unit 130 having the above-described configuration, the diameter of a portion through which the generally spherical tripod shaft portion 11A is passed (in this case, the inside diameter of the inner roller 134) is constant in the direction of a roller rotation axis Z130. As illustrated in FIG. 7A and FIG. 7B, an outside diameter D11 of the tripod shaft portion 11A is the maximum outside diameter of the generally spherical tripod shaft portion 11A at a position near the center of the tripod shaft portion 11A. The outside diameter D11 is set slightly smaller than an inside diameter D134 of the inner roller 134. Therefore, as illustrated in FIG. 7A and FIG. 7B, it is possible to fit the roller unit 130 onto the tripod shaft portion 11A from the snap ring 135 side (see FIG. 7A), and it is also possible to fit the roller unit 130 onto the tripod shaft portion 11A from the rib 131B side (see FIG. 7B). However, the roller unit 130 is not symmetrical in shape in the direction of the roller rotation axis Z130, as illustrated in FIG. 6, FIG. 7A and FIG. 7B. The rib 131B is located on the one end face side, and the ring groove 131A and the snap ring 135 are located on the other end face side. The rib 131B is formed at such a position as to be flush with the one end face. The snap ring 135 is provided at a position that is slightly offset from the other end face toward the rib 131B in the direction of the roller rotation axis Z130. The roller unit 130 is not symmetrical in shape in the direction of the roller rotation axis Z130. As a result, there are differences in, for example, strength and performance between the case where the roller unit 130 is fitted onto the tripod shaft portion 11A from the snap ring 135 side as illustrated in FIG. 7A, and the case where the roller unit 130 is fitted onto the tripod shaft portion 11A from the rib 131B side as illustrated in FIG. 7B. In the examples illustrated in FIG. 7A and FIG. 7B, in the case where the roller unit 130 is fitted onto the tripod shaft portion 11A from the snap ring 135 side as illustrated in FIG. 7A, the snap ring 135 does not come into contact with the tripod shaft portion 11A even if the tripod shaft portion 11A is largely tilted. On the other hand, in the case where the roller unit 130 is fitted onto the tripod shaft portion 11A from the rib 131B side as illustrated in FIG. 7B, there is a possibility that the tripod shaft portion 11A and the rib 131B will come into slight contact with each other if the tripod shaft portion 11A is largely tilted. Thus, there are differences in, for example, strength and performance between these two cases.
Japanese Patent Application Publication No. 7-151158 (JP 7-151158 A) describes a double-roller-type tripod constant-velocity joint that includes an inner roller and an outer roller. According to JP 7-151158 A, a tripod shaft portion is passed through a roller unit in which the outer roller is fitted on the inner roller, from one end face side of the roller unit, on which a pressure disc is provided. Japanese Patent Application Publication No. 2008-208858 (JP 2008-208858 A) describes a tripod constant-velocity joint that includes, instead of an inner roller, a cylindrical holder having a flange that holds an outer roller. An outer member of the tripod constant-velocity joint is compact and configured such that a resin boot is easily fitted to the outer member.
In the double-roller-type tripod constant-velocity joint, the roller unit 130 illustrated in FIG. 6 is not symmetrical in shape in the direction of the roller rotation axis Z130. In the case where the tripod shaft portion 11A is passed through the roller unit 130 from the snap ring 135 side as illustrated in FIG. 7A, strength and performance are higher than those in the case where the tripod shaft portion 11A is passed through the roller unit 130 from the rib 131B side as illustrated in FIG. 7B. Therefore, a configuration that allows the tripod shaft portion 11A to be passed through the roller unit 130 only from the snap ring 135 side is required. In the related art described in JP 7-151158 A, the tripod shaft portion is passed through the roller unit in which the outer roller is fitted on the inner roller, from the one end face side on which the pressure disc is provided. With this configuration, however, the tripod shaft portion may also be passed through the roller unit from the other end face side on which no pressure disc is provided. This may cause a possibility of wrong assembly. In the related art described in JP 2008-208858 A, the tripod shaft portion may be passed through the roller unit in which the outer roller is fitted to the holder, from either end face side. This may cause a possibility of wrong assembly. Note that neither JP 7-151158 A nor JP 2008-208858 A has description regarding a configuration that allows the roller unit to be fitted to the tripod shaft portion only from a specific end face side of the roller unit.